During the fabrication of large area flat panel displays, defects sometimes occur which render a display unacceptable in appearance. Both inter-level shorts and open lines can result in pixels which cannot be turned on. The fabrication of crossing conductors, such as row and column electrodes, and of transistors involves applying an insulating layer over one conductor and then applying a second conductor over the insulation. Pinholes in the insulation are a cause of shorts between the two conductors. Normally, ground potential is maintained on the row electrodes except during the addressing of a specific row. A short of a column electrode to such a grounded row electrode at one location can cause the non-operation of a large number of pixels in the same column, due to the suppression of the column electrical signals. Of course, open column electrodes as well as shorted transistors can also cause loss of pixel operation. To minimize the number of rejected display panels due to such defects and raise the yield of the displays, it is desirable to provide redundant circuits which permit operation of a given pixel even though another circuit to that pixel is faulty.
One proposal for providing such a redundant circuit is set forth in U.S. Pat. No. 4,820,222 issued to Holmberg et al which suggests using two row electrodes for each row and two column electrodes for each column and dividing each pixel into four sub-pixels which are normally turned on and off at the same time and yet if one sub-pixel were inoperative some of the others would be able to function normally to minimize the effect of the fault on the appearance of the pixel. While this strategy may be useful for coarse displays, it does not apply to fine resolution displays. The proposal is equivalent to using four pixels to do the job of one. Since there is a practical limit to the density of phosphor elements comprising pixels, especially in high quality bright displays, the proposed sub-pixel scheme has a density limit of one fourth the normal pixel density limit. The circuitry in that patent is effective only for liquid crystal (field effect) display devices whereas emissive displays are current driven and thus require circuits for each pixel which are effective to sustain current so long as the pixel is to be illuminated.